/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.streaming.examples.statemachine.dfa;

import org.apache.flink.streaming.examples.statemachine.event.EventType;

import java.util.Random;

/**
 * The State captures the main functionality of the state machine. It represents
 * a specific state in the state machine, and holds all transitions possible
 * from a specific state.
 *
 * <p>The state transition diagram is as follows:
 * <pre>
 *           +--[a]--> W --[b]--> Y --[e]---+
 *           |                    ^         |
 *   Initial-+                    |         |
 *           |                    |         +--> (Z)-----[g]---> Terminal
 *           +--[c]--> X --[b]----+         |
 *                     |                    |
 *                     +--------[d]---------+
 * </pre>
 */
public enum State {

	/**
	 * The terminal state in the state machine.
	 */
	Terminal,

	/**
	 * Special state returned by the State.transition(...) function when attempting
	 * an illegal state transition.
	 */
	InvalidTransition,

	/**
	 * State 'Z'.
	 */
	Z (new Transition(EventType.g, Terminal, 1.0f)),

	/**
	 * State 'Y'.
	 */
	Y (new Transition(EventType.e, Z, 1.0f)),

	/**
	 * State 'X'.
	 */
	X (new Transition(EventType.b, Y, 0.2f), new Transition(EventType.d, Z, 0.8f)),

	/**
	 * State 'W'.
	 */
	W (new Transition(EventType.b, Y, 1.0f)),

	/**
	 * The initial state from which all state sequences start.
	 */
	Initial(new Transition(EventType.a, W, 0.6f), new Transition(EventType.c, X, 0.4f));

	// ------------------------------------------------------------------------

	private final Transition[] transitions;

	State(Transition... transitions) {
		this.transitions = transitions;
	}

	/**
	 * Checks if this state is a terminal state.
	 * A terminal state has no outgoing transitions.
	 */
	public boolean isTerminal() {
		return transitions.length == 0;
	}

	// ------------------------------------------------------------------------

	/**
	 * Gets the state after transitioning from this state based on the given event.
	 * If the transition is valid, this returns the new state, and if this transition
	 * is illegal, it returns [[InvalidTransition]].
	 *
	 * @param evt The event that defined the transition.
	 * @return The new state, or [[InvalidTransition]].
	 */
	public State transition(EventType evt) {
		for (Transition t : transitions) {
			if (t.eventType() == evt) {
				return t.targetState();
			}
		}

		// no transition found
		return InvalidTransition;
	}

	/**
	 * Picks a random transition, based on the probabilities of the outgoing transitions
	 * of this state.
	 *
	 * @param rnd The random number generator to use.
	 * @return A pair of (transition event , new state).
	 */
	public EventTypeAndState randomTransition(Random rnd) {
		if (isTerminal()) {
			throw new RuntimeException("Cannot transition from state " + name());
		}
		else {
			final float p = rnd.nextFloat();
			float mass = 0.0f;
			Transition transition = null;

			for (Transition t : transitions) {
				mass += t.prob();
				if (p <= mass) {
					transition = t;
					break;
				}
			}

			assert transition != null;
			return new EventTypeAndState(transition.eventType(), transition.targetState());
		}
	}

	/**
	 * Returns an event type that, if applied as a transition on this state, will result
	 * in an illegal state transition.
	 *
	 * @param rnd The random number generator to use.
	 * @return And event type for an illegal state transition.
	 */
	public EventType randomInvalidTransition(Random rnd) {
		while (true) {
			EventType candidate = EventType.values()[rnd.nextInt(EventType.values().length)];
			if (transition(candidate) == InvalidTransition) {
				return candidate;
			}
		}
	}
}
